My nomination is Two Broke Girls.
I first saw Two Broke Girls on an airplane, I didn't have enough time before landing to start watching another movie, so I thought I'd watch a couple of episodes of a sitcom. I couldn't quite believe how bad it was, and when I got home I watched another episode just to make sure I hadn't hallucinated its awfulness in some kind of jet lagged haze. I hadn't.
If you haven't seen it it's like one of those dreadful Nickelodeon or Disney kids sitcoms where everything is overly bright and the programme makers have mistaken having the actors loudly shouting catch phrases for character development. Except as its made for adults most of the 'jokes' are crude sexual humour, which I don't mind when it's funny, but this isn't funny, it's the adult equivalent of a five year old shouting 'poo bum fart willy' at you constantly. Plus all the supporting characters are horrible racial stereotypes.
There must be a fan base for it because it's on about season four, but dear god it's bad.